The present invention relates to the field of differential amplifying circuits, preferably including transistor circuits in a CMOS technology.
In a growing number of applications of signal processing, the use of small analogue circuits is required. Small circuits limit the available type of transistors to small transistors, which almost inevitably results in offset problems from poor matching of pairing transistors that are needed as pairs in the circuits. In many applications the input signal is typically smaller (e.g. microvolts) than the input referred offset (e.g. tens of millivolts) of circuits such as an Operational Transconductance Amplifier (OTA) or Operational Amplifier (Opamp) of a preamplifier device. This induces the subsequent amplifier stages to go out of range.
Placing large capacitors in the signal path between amplifier stages is the generally known way in the art to solve this problem. It filters out the lower frequency components of the signal. Including those lateral capacitors in a CMOS circuit typically requires the use of an analogue CMOS technology (with dual polysilicon option). Moreover, to get the low corner frequency of the pass-band at a relative low frequency level, the capacitors need to be very large and occupy a relative large Si-area.
Apart herefrom, another common trend is to use fully symmetric amplifier circuits instead of single ended ones, and to choose for a fully differential signal treatment as a concept. One of the advantages of this strategy is the improved immunity to several kinds of electromagnetic interferences and to power supply ripple and spikes. The known way to the person skilled in the art is to make the OTA or Opamp with an explicit regulating loop to steer the common mode output voltage to a predetermined or predefined voltage level, typically to Vcc/2. Such extra feedback loop makes such a system more complex, larger and thereby requires extra power. Moreover, instead of placing one or more single large capacitors in the signal path, twice the number of these capacitors become required in the differential signal path.
Document US-A-5736892 describes a differential amplifier, which exhibits however no pass band characteristic.
It is an aim of the present invention to disclose a compact differential amplifying circuit.
It is another aim of the present invention to disclose a differential amplifying circuit with a built-in band-pass filtering feature and with regulation, preferably self-regulation, of the common mode output voltage of the differential amplifying circuit. The common mode output voltage is the average of the output voltages at the different output nodes of the differential amplifying circuit.
It is yet another aim of the present invention to disclose a differential amplifying circuit where the output voltage level, preferably the common mode voltage level, is regulated by the active load of the circuit and whereby the lower frequency components in the input signal and the input referred offset voltage can get less amplified than the useful frequency components.
Yet according to another aim of the invention, a differential amplifying circuit is disclosed such that it is immune to mismatches between pairing transistors of the circuit. Therefore it becomes possible to work with much smaller transistors and lower current levels, occupying less Si area, and dissipating less power than in prior art circuits.
The active loads in the two output branches of the differential amplifying circuit of the invention, preferably an Operational Transconductance Amplifier (OTA), can show a high conductance at low frequency and low conductance at higher frequency. In this way an OTA is constructed with inherent pass-band. Low frequencies are amplified little or even filtered out. In an embodiment of the invention wherein the amplifying elements of the amplifier circuit are transistors, the amplification of input referred offset voltage due to mismatches in transistor pairs is similarly reduced. Complementary, in another embodiment of the invention, the OTA is of the lowpass type, i.e. also amplifying DC signals. In both embodiments, both OTA""s are very compact and the common mode output voltage regulation is in both cases part of the active load subcircuit.
The terms high frequency and low frequency in the present specification, have a meaning that depends on the specific application wherein the circuit of the invention is used. Generally, with high frequency it is meant signals in the range of 100 MHz up to of the order of GHz or higher. Low frequency signals can range from a few Hz up to 100 MHz.
Thus the present invention is related to a differential amplifying circuit having at least two branches wherein an output node is present, each of said branches comprising a first amplifying element and a load element, said load element including a subcircuit for regulating the signal at the output node of the branch, wherein said subcircuit comprises a second amplifying element with a control terminal and two electrodes and wherein said control terminal is connected via an impedance element with one of said electrodes and wherein the one electrode is connected to said output node.
Preferably the one electrode is directly connected to said output node, i.e. with a metal line connection. Other circuit elements however can be in the connection between the one electrode and the output node. Such circuit elements can include elements such as a resistor element or a switch.
According to an embodiment of the invention, said subcircuit includes a frequency-dependent pass-characteristic.
According to a further embodiment of the invention, said subcircuit includes a low frequency pass filter.
According to a preferred embodiment of the invention, the amplifying elements and impedance elements comprise transistors. According to a further preferred embodiment of the invention, said impedance element comprises at least a transistor operating in the subthreshold regime or in the triode regime.
The control terminals of the two amplifying elements of the two branches of the circuit can be connected to a common node.
Said subcircuit may further comprise at least one additional impedance element, preferably capacitor, element being connected to the other electrode of at least one of the impedance elements of the branches.
Thus the common mode output voltage at the output node of the differential amplifying circuit is determined by the transistor parameters and the operating bias current of the differential amplifying circuit. The regulation of the output common mode is in this way inherently present in the OTA circuit.
In an embodiment of the invention, the differential amplifying circuit can be implemented in a CMOS technology.
In an embodiment of the invention, the branches of the differential amplifying circuit are fully symmetric.